The conventional method of overcoming the foregoing problem, is to employ a time gate coinciding with the time required for signals to return from flaws closely adjacent the back surface. A suitably large gain during this period permits their display. A second gate is then employed whose start is slaved to the end of the flaw monitoring gate and is of sufficient length to contain the back echo. During the time period occupied by this back echo gate, a gain reduction signal is fed to the receiver in order to bring the back echo into the receiver's linear range. The problem which arises, however, is that the rapid change in gain which is required to bring the back echo into linear range causes detrimental side effects such as spurious signals at the gate edges.
It is a primary object of the present invention to provide ultrasonic pulse echo method and apparatus for accurately observing both flaw signals and changes in the back echo signal. Other objects are to provide such method and apparatus which avoid saturation of the receiver, which avoid transients and spurious side effects, and which do not require complex electronics or apparatus. Other objects, features and advantages will become apparent from the following description and appended claims.